Statutory Information and Restrictions (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statutory Information and Restrictions [Line Items]
Statutory capital and surplus of insurance subsidiaries	$ 912,336				$ 883,870								$ 912,336	$ 883,870
Deferred policy acquisition costs	267,580				253,176								267,580	253,176
Difference in policyholder reserves	98,360				95,536								98,360	95,536
Goodwill	47,396				47,396								47,396	47,396
Investment fair value adjustments on fixed maturities	301,518				314,705								301,518	314,705
Difference in investment reserves	125,805				120,795								125,805	120,795
Federal income tax liability	(228,090)				(224,492)								(228,090)	(224,492)
Net funded status of pension and other postretirement benefit obligations	(18,250)				(18,213)								(18,250)	(18,213)
Non-admitted assets and other, net	22,888				21,691								22,888	21,691
Shareholders' equity of parent company and non-insurance subsidiaries	11,648				17,172								11,648	17,172
Parent company short-term and long-term debt	(247,209)				(246,975)								(247,209)	(246,975)
Shareholders' equity as reported herein	1,293,982				1,264,661				$ 1,336,463				1,293,982	1,264,661	$ 1,336,463
Statutory net income of insurance subsidiaries													74,574	87,619	97,875
Net loss of non-insurance companies													(5,135)	(4,474)	(3,906)
Interest expense													(11,808)	(13,122)	(14,198)
Debt retirement costs													0	(2,338)	0
Tax benefit of interest expense and other parent company current tax adjustments													5,637	6,829	6,371
Combined net income													63,268	74,514	86,142
Increase (decrease) due to:
Deferred policy acquisition costs													19,442	13,249	16,828
Policyholder benefits													14,919	14,065	15,284
Federal income tax expense													(5,312)	(6,678)	(10,548)
Investment reserves													(1,320)	7,339	3,574
Other adjustments, net													(7,232)	(9,007)	(7,037)
Net income as reported herein	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 30,068	$ 25,357	$ 20,452	$ 28,366	$ 83,765	$ 93,482	$ 104,243